1. Field of Invention
The present invention relates to a novel method of and apparatus for transferring heat using heat exchanging fluids that are safely isolated from the environment above and below the Earth's surface and circulated within a sealed heat exchanging structure so as to improve the heat transfer performance of aqueous-based fluid heat transfer systems, wherein the ground, a lake, a river, or sea water is used as the primary or secondary heat sink or heat source in the sealed heat exchanging structure.
2. Brief Description of the State of Knowledge in the Art
The development of refrigeration processes, associated equipment, and two-phase chemical refrigerants evolved primarily in response to mankind's need to preserve food. Over the years, several different kinds of heat transfer systems have been developed for dissipating heat removed from the food to the exterior of the food storage container.
One type of heat transfer system is a typical refrigeration system which includes an evaporator for absorbing heat from one location, a condenser for dissipating heat to another location, a compressor for compressing the vaporous two-phase refrigerant exiting the evaporator for delivery into the condenser where the refrigerant is condensed back into a liquid, and a two-phase throttling device connected to the evaporator inlet for receiving the liquid refrigerant and refrigerant expansion, to complete a refrigeration cycle.
Condensers can be constructed in various configurations, namely: as an arrangement of tubing with air-cooled fins, or as a water-cooled “tube and shell configuration”.
In FIGS. 1 and 1A, the fluid flow characteristics of conventional tubing used in heat exchangers is schematically depicted. FIG. 1 illustrates how the velocity of a fluid traveling through a tube decreases from the center of the tube towards the inner surface of the tube. As shown, fluid enters tube A through inlet opening C. A laminar fluid flow profile D is caused by friction along a boundary layer E on the inner surface of the tube A. In the general, the annular region of flow B does not contain eddy currents during laminar flow. The shape of the laminar fluid flow profile D is influenced by the viscosity of the fluid passing through the tube. Fluids having lower viscosities cause a thinner boundary layer E to form, whereas fluids with higher viscosities, such as propylene glycol mixture (anti-freeze) and other viscosity increasing additives, causes a thicker boundary layer E to form, which reduces heat transfer. Fluids with lower viscosities, such as pure water, can transition into turbulent flow at lower fluid velocities.
Turbulence can be regarded as a highly disordered motion of matter (e.g. water, air etc.) resulting from the growth of instabilities in an initially laminar flow, and it is generally agreed that the transition from laminar to turbulent flow may be described as a series of events that take place more or less continuously. Additionally, it is known that turbulent fluid flow characteristics, due to eddy currents, can increase heat transfer across heat conducting surfaces. During turbulent flow, the annular flow region contains eddy currents which increase heat transfer. Fluids with higher viscosities, such as propylene-glycol, require higher fluid velocities in order to transition into turbulent flow. This requires more pump energy to transfer the same quantity of heat energy as pure water. Also, tubing with a rough inner surface allows the fluid to transition into turbulent flow at a lower fluid velocity.
FIG. 1A illustrates how flow velocities within annular regions of fluid flow B, C and D are influenced by frictional forces generated from boundary layer E. Within the boundary layer E, against the inner surface of the tube, the velocity of the heat transfer fluid can be zero. In the annular flow region B, the fluid is shown moving at 3 feet per minute, and faster in areas closer toward the annular flow region C, which has a flow velocity of 3 feet per minute. The fluid in the annular flow region D has the highest velocity of 10 feet per minute. Typically, the reduction in fluid flow velocity adjacent the inside surface of a tube results in a decrease in the rate of heat energy transferred from the fluid into the tube material.
The primary method of compensating for such heat transfer constraints (imposed by laminar fluid flows through conventional tubing structures) has been to use larger tubes and more powerful pumps in conventional heat transfer systems, which has resulted in higher installation costs at lower operating efficiencies.
At this juncture, it is appropriate to continue surveying prior art systems with such considerations in mind.
In the water-cooled tube and shell condenser, the rate of heat transfer between the refrigerant in the refrigeration-sealed system, and the water flowing around the tube and shell condenser tube, is much higher than the rate of heat transfer between the refrigerant in the refrigeration-sealed system and air flowing around the tubes of the air-cooled fin and tube condenser. A water-cooled tube and shell condenser is normally connected with pipes to a cooling tower and a water pump. The heat is absorbed by the water while circulated through the condenser. The heat in the water entering the cooling tower is then dissipated into the atmosphere from the water, completing a closed-loop water-cooled refrigeration process.
Environmental concerns have caused strict restrictions to be placed on water-cooled tube and shell condenser systems utilizing a water pump to gather water from natural sources such as a lake, a river, sea water, and other fluid systems, for circulation through the water-cooled tube and shell condenser of such heat transfer systems. Environmental contaminations vary but are mostly related to chemical concentrations and temperature variations being dispensed into the water source.
A water-cooled tube and shell condenser can be connected to a ground-source heat transfer well using pipes, to dissipate heat into the Earth. In various manufacturing processes, the required operating temperature and capacity (i.e. volume) of heat transfer fluid circulated through the ground source heat transfer well, may not require adding refrigeration to the system.
Ground loop heat transfer installations vary from trenched horizontal loops to multiple vertical loops. In FIGS. 1B and 1C, vertical installations are schematically illustrated in two different heat transfer modes.
In FIG. 1B, a (field assembled) conventional “U” tube type heat transfer tube is shown buried in the Earth for the purpose of dissipating heat energy from the system into the Earth. Typically, tube sections G and I are buried beneath the Earth a few inches apart from one another. During operation, a heat transfer fluid flows into inlet F in a laminar manner at 110 degrees Fahrenheit, and is forced to flow down tube section G. Heat energy in the laminar flowing fluid is transferred into the Earth at 55 degrees Fahrenheit, along the entire outer surface of the tube G. As illustrated, a portion of the heat from tube section G is actually transferred into tube section I after the heat transfer fluid has reversed its flow direction when flowing along elbow section H. Using this ground loop arrangement, the net amount of heat energy actually transferred into the Earth is diminished due to the heat transfer from tube section G into tube section I. Thus, the overall heat transfer capacity offered by this system design is significantly diminished due to (i) the laminar flow profile of the heat transfer fluid within the “U” tube construction (illustrated in FIG. 1), and (ii) the commingling of heat energy exchanges between underground tube sections G and tube I.
In FIG. 1C, a (field assembled) conventional “U” tube type heat transfer tube is shown buried in the Earth for the purposes of collecting heat energy therefrom. In this configuration, a heat transfer fluid flows at 40 degrees Fahrenheit into inlet F in a laminar manner, and is forced to flow down tube section G into the Earth. Along the entire outer surface of the tube section G, heat energy is transferred from within the 55 degree Fahrenheit Earth, into the heat transfer fluid maintained at 40 degree Fahrenheit. Since the 15 degree Fahrenheit temperature difference between the heat transfer fluid inside tube section G is higher than that of the heat transfer fluid occupying tube section I, more heat energy is absorbed by tube section G than is absorbed by tube section I. Also, a portion of the heat energy transferring into tube section G originates from tube section 4 and is actually transferred into tube section G after the heat transfer fluid has reversed its flow direction along elbow section H. Again, the overall heat transfer capacity offered by this system design is significantly diminished due to (i) the laminar flow profile of the heat transfer fluid within the “U” tube construction (illustrated in FIG. 1), and (ii) the commingling of heat energy exchanges between underground tube sections G and tube I.
Residential and commercial comfort air conditioning systems using “air-cooled condensers” are also well known in the art. Air-cooled condensers are also used extensively world-wide on air conditioners employing heat pumps. In contrast, “water-cooled tube” and shell condensers are typically used in large tonnage commercial and industrial applications such as in high-rise buildings, natural gas dehydration, and liquefied natural gas gasification systems.
A heat pump, originally called a reverse refrigeration system, reverses the refrigeration process through the use of sealed system valves and controls causing the evaporator to dissipate heat while causing the condenser to absorb heat. In its cooling mode of operation, an air conditioning system employing a ground-source heat-pump will dissipate heat into the Earth while, and absorb heat from the Earth in its heating mode of operation.
Over the years, the ground/water source type heat pump has proven very useful as a very efficient form of heating and cooling technology. The use of ground/water source type heat pumps have three distinct advantages over air source type heat pumps, namely: during the peak cooling and heating seasons, the ground/water source usually has a more favorable temperature difference than the atmospheric air; the liquid-refrigerant exchanger on the heat pump permits a closer temperature approach than an air-refrigerant exchanger; and there is no concern with frost/snow/ice/dirt buildup or removal on the heat exchanger.
In general, prior art heat pump installations have employed undersized ground loops (constructed using conventional U type tubing) because refrigerant-based fluids can provide a sufficient temperature difference between the fluid and the ground so that enough heat is transferred to and from the ground to match the heating/cooling load on the heat pump. However, the use of undersized ground loops is also known to reduce the SEER rating of the heat-pump system. Also, the design goals of prior art heat pump systems have been to minimize the length of the metal pipe (i.e. tubing) used in the ground loop, while just passing the minimum standards for efficiency.
When prior art heat pump systems experience peaks or spikes in heating/cooling loads during daily operations, thermal storage solutions are oftentimes added to the system in order to average the load over the time period of interest. Thermal storage solutions also help reduce the cost of the ground loop by allowing the loop to be sized for the average base load over the day, week or season. In fact, many large buildings and residences use thermal storage solutions in order to reduce the cost of heating and cooling by (i) using less expensive night-time electrical loads to heat/cool the thermal mass, and then (ii) using the thermal mass to heat/cool the building during the day. In order to reduce capital cost of the heat pump system, prior art heat pump system installations often use the metal rebar in the foundation or piling as a major part of the thermal mass of the ground loop portion of the heat pump system.
Ground source or water source type heat pumps can use a closed or open loop as a heat exchanger. Open loops include water circulated to cooling towers; water circulated between wells, geothermal steam wells, water circulated in a body of water such as a river or lake. Closed loops include aqueous-based fluids and refrigerant-based fluids circulated in cooling/heating coils that transfer heat to air, water, and ground. Most power plants use at least one open loop to generate steam (the burner exhaust) and one open loop (cooling towers or lake) to condense the steam back to water. The de-ionized steam source water is preserved in a close loop to prevent scale buildup in the heat exchanger. Most conventional refrigerators, freezers and air conditioners use a closed loop of refrigerant to cool the load and an open loop of external air to condense the refrigerant.
The shortcomings and drawbacks of using air to transfer heat from the condenser coil is that air requires a high temperature differential and a large condenser coil surface area to achieve reasonable heat transfer rates. The high temperature differentials translate to a high-pressure differential which implies higher energy costs to transfer a unit of heat. When a heat pump uses a liquid, from a water or ground loop, to transfer heat from the condenser coil, a smaller coil and a lower temperature and pressure differential can be used to transfer the same unit of heat as the air cool condenser coil which, in turn, improves efficiency and reduces energy costs.
When closed loops are used in the ground or water source of a heat pump system, there is a trade off between using (i) metal tubing with a high heat transfer coefficient (i.e. which is subject to corrosion and thermal expansion), and (ii) plastic tubing with a low heat transfer coefficient, which is resistant to corrosion and thermal expansion. For average soil conditions, plastic tubing usually will require about three (3) times the heat transfer area of the metal tubing to maintain an equivalent heat transfer rate. Metal tubing is usually reserved for refrigerant-based fluids due to the high fill pressures and the reactivity of the refrigerant with plastic tubing.
While protective coatings and grouting can reduce the corrosion rates of metal tubing, pin holes in the coating or grout can actually concentrate the anode corrosion rate in the pin-hole area. Electrical measurements have shown that circulating aqueous based fluids between the ground loop and heat pump can cause the flow of a low level current between the building and the ground.
In accordance with convention, a close-loop ground/water source heat pump can use a refrigerant based fluid or an aqueous-based fluid. With refrigerant-based fluids, the heat pump can use a high differential temperature to transfer heat between the ground and the fluid in the tubing, but extra energy load reduces the SEER rating of the heat pump system. Metal tubing is used to contain the pressurized refrigerant-based fluid and minimize the volume of refrigerant in the ground loop system due to the high heat transfer coefficient of the metal.
As discussed in U.S. Pat. No. 5,025,634 to Dressler, refrigerant based fluids have very high maintenance cost when a small leak develops in the ground/water loop and a very high environmental impact when there is a release of the refrigerant. Also, over a long period of time, field experience has shown that high pressure head loss can develop in the closed ground/water source loop when lubricating oil from the compressor collects low spots in horizontal loop or at the bottom of the bore hole in vertical loop.
With most aqueous-based fluid ground/water source loops, the heat pump uses a small close-loop refrigerant heat exchanger to transfer heat to or from the aqueous fluid. The small heat exchanger reduces the capital cost of the heat pump and reduces the chances of refrigerant releases to the environment. In areas with ground movement, such as earthquakes zones, subsidence bowls, and deep freeze/thaw zones, the borehole thermally-conductive outer tube and transfer piping can develop leaks due to repeated damage over time as discussed in U.S. Pat. No. 4,993,483 to Kurolwa.
As disclosed in U.S. Pat. No. 4,644,750 to Lockett and Thurston and in U.S. Pat. No. 4,325,228 to Wolf, a horizontal ground loop's performance is affected by fluctuation in atmospheric surface temperature and soil moisture content, whereas, the ground loop based on multiple bore holes has a stable fluid temperature and heat transfer coefficient for both heating and cooling thermal loads. For heat and cooling loads located on small land surfaces or arid land, the ground loop heat exchanger based on multiple bore holes can provide a heat pump with a stable heat sink or source as described in U.S. Pat. No. 4,392,531 to Ippolito.
The first major improvements to ground loop fluid heat transfer using metal tubing and refrigerant based fluids are disclosed in U.S. Pat. No. 5,816,314 to Wiggs et. al, U.S. Pat. No. 5,623,986 to Wiggs, U.S. Pat. No. 5,461,876 to Dressler, U.S. Pat. No. 4,867,229 to Mogensen, and U.S. Pat. No. 4,741,388 by Kurolwa where metal tubing was bent into a helix shape to increase heat transfer between the refrigerant and the ground. These five patents disclose that the ‘vertical helical heat exchanger’ or the ‘bore-hole helical heat exchanger’ provides the heat pump with a stable heat sink or source for heating and cooling. The shortcoming of these designs is the increased capital cost of helical bending of the tubing and the increased installation cost involved in running bent helical tubing in a deviated well.
Another popular technique used in prior art heat pumps involves insulating the metal, fluid-return tube from the bottom of the bore hole so to prevent heat transfer from incoming fluid, which significantly improves the heat exchanger performance. The deficiency of such prior art insulating methods has caused a significant increase in installation costs and a significant increase in capital cost associated with insulating materials. Notably, as the return line was far enough away from the loop to not cause any significant thermal interference, insulating the fluid return tube was not required for earlier horizontal ground loop heat exchangers.
U.S. Pat. No. 5,623,986 to Wiggs also discloses that external helically shape fins can be used to drill short vertical heat exchangers into sand-loam soils or mud bottoms, but field experience has shown that there is too much fin damage when installing vertical heat exchangers in hard rock/ground surfaces.
U.S. Pat. No. 5,937,665 to Kiessel et al., discloses other improvements to refrigerant based groundloops, wherein an air heat exchanger is used to the system to reduce the load on the ground loop.
U.S. Pat. No. 6,138,744 by Coffee discloses using a large storage tank of water to a horizontal ground loop that is continuously replenished by an external water source such as water well. This technique involves combining an open water loop and a lose ground loop.
U.S. Pat. No. 6,615,601 by Wiggs discloses combining a solar heating loop and a water evaporative cooling loop to the ground loop so as to supplement the heating and cooling load.
U.S. Pat. No 6,212,896 to Genung discloses a ground loop with large well bores to make room for a vertical thermal siphon to enhance the heat transfer in the large well bore. The shortcoming of this idea is that the heat is transfer to the thermally-conductive outer tube wall with a laminar flow of fluid.
U.S. Pat. No. 6,672,371 to Amerman et al. created a ground loop by drilling multiple well bores from one pad and using plastic U-tubes for the heat exchanger. By using many plastic U-tubes with low heat transfer in series, an equivalent metal heat exchanger performance can be achieved in the ground loop.
Also, U.S. Pat. No. 6,789,608 to Wiggs discloses a technique for extending the performance of the U-tube heat exchanger by installing an insulating plate between the tubes to make two close separate half wells with minimal thermal interference between each well.
Thus, while various advances have been made in heat transfer system design and implementation, there is still a great need in the art for an improved method of and apparatus for transferring heat from above or below the Earth's surface using a sealed fluid circulation system, while overcoming the shortcomings and drawbacks of prior art methodologies and equipment.